In geometry, a set of points are said to be concyclic (or cocyclic) if they lie on a common circle. All concyclic points are the same distance from the center of the circle. Three points in the plane that do not all fall on a straight line are concyclic, but four or more such points in the plane are not necessarily concyclic.

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In general the centre O of a circle on which points P and Q lie must be such that OP and OQ are equal distances. Therefore O must lie on the perpendicular bisector of the line segment PQ.[1] For n distinct points there are n(n − 1)/2 bisectors, and the concyclic condition is that they all meet in a single point, the centre O.

The vertices of every triangle fall on a circle. (Because of this, some authors define "concyclic" only in the context of four or more points on a circle.)[2] The circle containing the vertices of a triangle is called the circumscribed circle of the triangle. Several other sets of points defined from a triangle are also concyclic, with different circles; see nine-point circle[3] and Lester's theorem.[4]

The radius of the circle on which lie a set of points is, by definition, the radius of the circumcircle of any triangle with vertices at any three of those points. If the pairwise distances among three of the points are a, b, and c, then the circle's radius is

The equation of the circumcircle of a triangle, and expressions for the radius and the coordinates of the circle's center, in terms of the Cartesian coordinates of the vertices are given here and here.

A quadrilateral ABCD with concyclic vertices is called a cyclic quadrilateral; this happens if and only if ∠CAD=∠CBD{\displaystyle \angle CAD=\angle CBD} (the inscribed angle theorem) which is true if and only if the opposite angles inside the quadrilateral are supplementary.[5] A cyclic quadrilateral with successive sides a, b, c, d and semiperimeters = (a+b+c+d)/2 has its circumradius given by[6][7]

an expression that was derived by the Indian mathematician Vatasseri Parameshvara in the 15th century.

By Ptolemy's theorem, if a quadrilateral is given by the pairwise distances between its four vertices A, B, C, and D in order, then it is cyclic if and only if the product of the diagonals equals the sum of the products of opposite sides:

AC⋅BD=AB⋅CD+BC⋅AD.{\displaystyle AC\cdot BD=AB\cdot CD+BC\cdot AD.}

If two lines, one containing segment AC and the other containing segment BD, intersect at X, then the four points A, B, C, D are concyclic if and only if[8]

AX⋅XC=BX⋅XD.{\displaystyle \displaystyle AX\cdot XC=BX\cdot XD.}

The intersection X may be internal or external to the circle. This theorem is known as power of a point.

Some authors consider collinear points (sets of points all belonging to a single line) to be a special case of concyclic points, with the line being viewed as a circle of infinite radius. This point of view is helpful, for instance, when studying inversion through a circle and Möbius transformations, as these transformations preserve the concyclicity of points only in this extended sense.[10]

In any triangle all of the following nine points are concyclic on what is called the nine-point circle: the midpoints of the three edges, the feet of the three altitudes, and the points halfway between the orthocenter and each of the three vertices.

If lines are drawn through the Lemoine pointparallel to the sides of a triangle, then the six points of intersection of the lines and the sides of the triangle are concyclic, in what is called the Lemoine circle.

The van Lamoen circle associated with any given triangle T{\displaystyle T} contains the circumcenters of the six triangles that are defined inside T{\displaystyle T} by its three medians.

A convex quadrilateral is orthodiagonal (has perpendicular diagonals) if and only if the midpoints of the sides and the feet of the four altitudes are eight concyclic points, on what is called the eight-point circle.